Optical readers, such as bar code readers, are now quite common. Typically, a bar code comprises a series of encoded symbols, and each symbol consists of a series of light and dark regions, typically in the form of rectangles. The widths of the dark regions, the bars, and/or the widths of the light spaces between the bars indicate the encoded information.
A bar code reader illuminates the code and senses light reflected from the code to detect the widths and spacings of the code symbols and derive the encoded data. These widths and spacings determine the density of the symbol. The denser the symbol, the greater the information which can be encoded within a particular area i.e. within the area of a symbol of a particular size. Symbols with bars and spaces of a width less than 7.5 mils, and more particularly in a range between 5.5 and 7.5 mils are generally considered to be high density symbols. Symbols having bars and spaces of widths over 10 mils are generally considered low density symbols. Accordingly, symbols with bar and space widths over 7.5 mils and up to 10 mils are considered medium density symbols. Optical scanning devices are often characterized on the basis of the density of the symbol which can be read within a particular range of distances to the indicia.
Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc. To achieve these advantages, however, users or employees must be willing to consistently use the bar code readers. The readers therefore must be easy and convenient to operate.
A variety of scanning devices are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols. Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,760,248--which are owned by the assignee of the instant invention and are incorporated by reference herein--have generally been designed to read indicia having parts of different light reflectivity, e.g., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working range or reading distance from a hand-held or stationary scanner.
FIG. 1 illustrates an example of a prior art bar code reader unit 10 implemented as a gun shaped device, having a pistol-grip type of handle 53. A lightweight plastic housing 55 contains the laser light source 46, the detector 58, the optics and signal processing circuitry and the CPU 40, as well as a power source or battery 62. A light-transmissive window 56 in the front end of the housing 55 allows the outgoing light beam 51 to exit and the incoming reflected light 52 to enter. The user aims the reader 10 at a bar code symbol 70 from a position in which the reader 10 is spaced from the symbol, i.e., not touching-the symbol or moving across the symbol.
As further depicted in FIG. 1, the reader 10 may include a suitable lens 57 (or multiple lens system) to focus the scanned beam into a scanning spot at an appropriate reference plane. The light source 46, such as a semiconductor laser diode, introduces a light beam into the axis of the lens 57, and the beam passes through a partially-silvered mirror 47 and other lenses or beam-shaping structures as needed. The beam is reflected from an oscillating mirror 59 which is coupled to a scanning motor 60 which is energized when the trigger 54 is pulled. The oscillation of the mirror 59 causes the reflected beam 51 to scan back and forth in a desired pattern.
A variety of mirror and motor configurations can be used to move the beam in a desired scanning pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the multi-mirror construction is mounted.
The light 52 reflected back by the symbol 70 passes back through the window 56 for application to the detector 58. In the exemplary reader 10 shown in FIG. 1, the reflected light reflects off of mirror 59 and partially-silvered mirror 47 and impacts on the light sensitive detector 58. The detector 58 produces an analog signal proportional to the intensity of the reflected light 52.
A digitizer circuit mounted on board 61 processes the analog signal from detector 58 to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer is applied to a decoder, typically a programmed microprocessor 40 which will have associated program memory and random access data memory. The microprocessor decoder 40 first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. This recognition of the standard is typically referred to as autodiscrimination.
To scan a symbol 70, a user aims the bar code reader unit 10 and operates movable trigger switch 54 to activate the light beam 51, the scanning motor 60 and the detector circuitry. If the scanning beam is visible, the operator can see the scan pattern on the surface on which the symbol appears and adjust aiming of the reader 10 accordingly. If the light produced by the source 46 is marginally visible, an aiming light may be included in the optical system. The aiming light if needed, produces a visible-light spot which may be fixed, or scanned just like the laser beam; the user employs this visible light to aim the reader unit at the symbol before pulling the trigger.
The reader 10 may also function as a portable computer terminal. If so, the bar code reader 10 would include a keyboard 48 and a display 49, such as described in the previously noted U.S. Pat. No. 4,409,470.
In optical scanners of the type discussed above, the laser diode, the lens, the mirror and the means to oscillate the mirror all add size and weight to the handheld scanner. The photodetector and the associated processing circuitry also add size and weight. In applications involving protracted use, a large heavy handheld unit can produce fatigue. When use of the scanner produces fatigue or is in some other way inconvenient, the user is reluctant to operate the scanner. Any reluctance to consistently use the scanner defeats the data gathering purposes for which bar code systems are intended.
The distance from which a scanner can read indicia may affect its usefulness. For example, it is often desirable for a user to scan indicia from long range, say up to five plus feet from the symbol for low density symbols. For high density symbols it is desirable if the user can read the symbol at no less than six plus inches from the target. However, signal to noise ratios at these ranges, have previously made it difficult to read high density symbols, for example those with bar/space widths of 5.5 to 7.5 mils or to read even low density symbols such as those with bar/space widths of 10 mil or more at such ranges, without paying a significant price in terms of the size, weight, operational life between recharging or changing the power supply, and hence in terms of the portability and operability, of the scanner.
It is beneficial to modularize scanning components, so that a particular module can be used in a variety of different scanners. A particularly small, light weight module-which contains all necessary scanner components would be particularly beneficial since this would allow all necessary components to be easily replaced by removing and installing a single small module.
Smaller size scanning components tend to operate at higher scanning frequencies. In typical bar code scanning applications, however, the scanning frequency of the moving spot should be relatively low, typically 20 Hz or less. If the frequency increases, the speed of the spot as it passes over the indicia increases. The signals produced by the detector also increase in frequency, and consequently the bandwidth of the processing circuitry for analyzing the detector signals must be increased. Also, operation at higher scanning frequencies generally produces detector signals which include higher levels of noise, making accurate decoding more difficult.